Heretofore, there had been suggested an air injection system as an apparatus for cleaning exhaust gases of an automobile, wherein secondary air is injected and fed into an exhaust path near the exhaust valve of an engine, and sufficient oxygen is provided while exhaust gases are in a high temperature state so that detrimental components in the exhaust gases may be oxidized and made nondetrimental.
In this cleaning system, in order to obtain the best cleaning rate, it is necessary to feed a proper amount of secondary air in response to the load on the engine. With an air pump driven generally by the engine to feed such secondary air, the amount of fed air is proportional to the number of revolutions of the engine, but has nothing to do with the load condition (the speed of the automobile) of the engine. Thus, the optimum amount of secondary air cannot be said to be always proportional to the number of revolutions of the engine. Further, at the time of a continuous load (at the time of a high speed running), the secondary air will become excessive, and the exhaust system will be overheated so much as to impair the function and obstruct the cleaning at the time of a low load.
Therefore, there is required a secondary air controlling apparatus wherein a proper amount of secondary air is fed to a system of feeding secondary air to an exhaust path so that the best cleaning may be obtained in response to the engine load, and wherein the secondary air is interrupted to protect the exhaust system at the time of continuous high load (at the time of a continuous high speed operation).
Generally, in an engine for a four-wheeled automobile having multi-cylinders with a single carburetor, the negative pressure in the suction pipe is high and stably varies with the load condition (car speed) as shown by the line H in the graph in FIG. 1. Therefore, a type wherein a control valve for controlling the amount of secondary air fed is operated by such negative pressure in the suction pipe has already been adopted as a secondary air controlling apparatus. The control valve is controlled with such negative pressure in the suction pipe so that secondary air may be fed in response to the load condition of the engine, and may be interrupted with the control valve to protect the exhaust system at the time of a continuous high load operation.
However, in an engine for a motorcycle or autobicycle including a system having a single cylinder with a single carburetor, the suction pipe negative pressure is unstable in the load condition as evident by the line I in the graph in FIG. 1 and, at the time of a high load (at the time of a high speed), the negative pressure will be so low that, in order to make a boost control, the diaphragm for making it will have to be very large. Therefore, the secondary air controlling apparatus would have to be so large as to be difficult to fit in a motorcycle or autobicycle which is restricted as to space, to cause such a serious design problem in case it is to be so fitted, to be high in cost, and to be difficult to adopt.